1. Field of the Invention
The present invention relates to the fabrication of semiconductor integrated circuit structures and, in particular, to the utilization of selective laser recrystallization for the formation of uniform emitter/base junctions and highly uniformly doped emitter electrodes in bipolar transistor structures.
2. Discussion of the Related Art
FIG. 1 illustrates several problems resulting from the formation of the emitters of bipolar transistors utilizing conventional integrated circuit fabrication techniques. First, variations in the vertical thickness of the emitter polysilicon film in the emitter window causes non-uniform doping of the emitter poly; this results in a non-uniform doping profile in the emitter region when ion implantation is used, which causes process non-uniformity and, therefore, device performance variations. Second, incomplete filling of the emitter window with emitter polysilicon creates voids, resulting in a non-uniform emitter/base contact interface due to emitter narrowing; this leads to a smaller and non-reproducible emitter/base junction interfacial area. Third, insufficient dopant activation using conventional annealing techniques results in lower current gain in the bipolar device.
The present invention utilizes selective laser recrystallization of emitter polysilicon for the formation of uniform emitter/base junctions and highly uniformly doped emitter electrodes using conventional ion implant doping and polysilicon deposition techniques.
Thus, the present invention is directed to a method of forming an emitter structure in a semiconductor integrated circuit bipolar transistor structure. The bipolar transistor structure includes a collector region of a first conductivity type formed in a semiconductor substrate and a base region of a second conductivity type, opposite the first conductivity type, formed in the collector region. A layer of dielectric material is formed on the surface of the base region. An emitter window is then opened in the dielectric material to expose a surface area of the base region. Conductive material is then formed over the layer of dielectric material and extending into the emitter window such that at least a portion of the layer of conductive material is in contact with the surface area of the base region. Dopant of the first conductivity type is then introduced into the layer of conductive material. A region of anti-reflective coating (ARC) material is formed on the layer of conductive material over the emitter window opening such that portions of the layer of the conductive material are exposed. Sufficient laser energy is then applied to the structure resulting from the foregoing steps to cause the conductive material underlying the region of anti-reflective coating to melt and flow. The anti-reflective coating material is then removed and the laser annealed conductive material is patterned to define an emitter region that extends into the emitter window opening and in interfacial contact with the surface area of the base region.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings which set forth an illustrative embodiment in which the principles of the invention are utilized.